This invention is related to line isolation monitor (LIM) systems for detecting the leakage current to ground in an isolated power system, and more specifically to those line isolation monitor systems which impress a signal into the isolated power system to determine a particular parameter of said power system.
In recent years, hospitals have found it desirable to use isolated power supplies in areas where there is a danger of explosion from gases or shock to hospital personnel and patients from various electrical equipment, such as in operating rooms or intensive care units. Theoretically, the isolated power systems have no ground connection so that in the event one should come into contact with a power line in this power system, no power would flow to ground through this person and a shock would be prevented. Furthermore, the possibility of creating a spark which could ignite any of the explosive gases commonly found in a hospital would be significantly reduced.
Regulatory agencies, such as the National Fire Prevention Association (NFPA), have set standards for isolated power supply systems in hospitals. Today, the standard generally applied is that the maximum total leakage, or hazard current, from the isolated power supply system cannot exceed 2mA. Values greater than this are presumed to indicate the existence of a ground fault in the isolated power system which could be sufficient to permit completion of the circuit in the above example, thereby increasing the risk that a spark could be created or someone could be shocked.
There are many types of line isolation monitor systems disclosed in the prior art which give warnings when the hazard current of the isolated power supply system rises above a predetermined amount. There are several problems characteristic of those disclosed in the prior art. Some line isolation monitor systems inject a test signal into the isolated power supply which can cause severe interference with the other hospital electronic equipment connected to the isolated power supply. Others use switching techniques for sampling the power signal from the isolated system, which can introduce significant interference into the electronic hospital equipment. Still other systems introduce such a large amount of hazard current that the amount of hazard current from the isolated power system which could normally be accepted is significantly reduced. Another problem with some monitor systems is that their operation is adversely affected by electrical interference from certain hospital equipment so that the value of total hazard current indicated is not reliable. Other problems incurred with prior art line isolation monitor systems include inaccurate readings and slow response times to such a degree that the monitor system itself constitutes a safety hazard.